A bipolar transistor is one type of device constituting a modern large-scale integrated circuit. Due to their high operating speed, large output current density, and small variation of turn-on voltage, bipolar transistors are suitable for analog circuits.
Performance requirements for semiconductor devices have increased with the steady development of semiconductor processes. Generally, in a traditional process for manufacturing a bipolar transistor (e.g., a vertical NPN transistor), an effective width of a base may be controlled by a two-step base/emitter thermal process. First, boron ions are implanted and diffused in a substrate to form a base region. Then phosphor ions are implanted and diffused in the base region to form an emitter region. The depth difference between a bottom of the base region and a bottom of the emitter region defines a width of the base.
FIGS. 1-3 illustrate a conventional process for manufacturing a vertical NPN transistor. As shown in FIG. 1, a semiconductor substrate 100 is provided. Suitable material for semiconductor substrate 100 may be, for example, silicon or silicon germanium. Antimony ions are then implanted and diffused in the semiconductor substrate 100 to form an N-type buried layer 101. Then, an N-type epitaxial layer 102 is formed on the N-type buried layer 101 using an epitaxial method.
A first photoresist layer (not shown) is then formed on the N-type epitaxial layer 102 and patterned by a photolithography process to define an opening in the first photoresist layer. Using the patterned first photoresist layer as an implantation mask, P-type ions are implanted and diffused in the N-type epitaxial layer 102 to form a base region 104 below the opening in the first photoresist layer. The P-type ions may be, for example, boron ions. After forming the base region 104, the first photoresist layer is removed, as shown in FIG. 2.
Referring to FIG. 3, a second photoresist layer (not shown) is formed on the N-type epitaxial layer 102. The second photoresist layer is patterned by a photolithography process to define an opening in the second photoresist layer. Using the patterned second photoresist layer as an implantation mask, N-type ions are implanted and diffused in the base region 104, so as to form an emitter 106. The remaining portion of the base region 104 not converted into P-type serves as the base of the transistor. The depth of the base region 104 is larger than that of the emitter 106.
Since the emitter in an NPN transistor formed by the conventional process is surrounded by the base, an edge-crowding effect of emitter current may occur. This effect may increase a current density at an edge of the emitter, resulting in a conductive modulation effect in the base. Moreover, the edge-crowding effect in the emitter may also cause a reduction of current density in a center region of the emitter, so that the area of the emitter may not be effectively utilized.